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3.0 Scientific Basis And Underlying Principles 3-8 April 2004 3.5 Role Of Floodplains Floodplains are low lying areas that adjoin the active river channel and become inundated during periods of overbank flooding (Armantrout 1998), primarily during spring floods. The reproductive biology of the razorback sucker is linked to these spring flood events (Tyus 1987; Tyus and Karp 1990; Modde et al. 1995). Adults deposit and fertilize eggs over main channel cobble bars near the peak of spring runoff (Tyus 1987; Tyus and Karp 1990; Modde and Irving 1998). Spawning occurs at 16-190C, hatching occurs in 6-7 days at 18-20oC, and larvae swim up in 12-13 days (Snyder and Muth 1990). Larvae become transported downstream by river currents at swim-up phase and are entrained in riverside floodplains when the river is still at flood stage (Osmundson and Kaeding 1989). These floodplain habitats are highly productive (Mabey and Shiozawa 1993; Modde 1997) and provide an important and timely food source for the young fish during a "critical period" when nutritional needs shift from endogenous (yolk) to exogenous (zooplankton) sources at between 8 and 19 days of age (Papoulias and Minckley 1990, 1992). Hence, it is critical for razorback sucker larvae to reach productive and sheltered habitats within 1- 2 days of swim-up (Figure 3-3). Several factors determine benefits of these floodplains to razorback sucker, induding timing of inundation, duration of intra-annual connection with the river, inter-annual frequency of connection, and inter- and intra-annual persistence of water quantity and quality to sustain fish. These factors are examined for each floodplain site described in this Plan, and actions are identified to maximize benefits to razorback sucker and possibly bonytail. Life history requirements ofbonytail are not well known, and it is hypothesized that an increased availability of floodplains will also benefit this species. <Eggs hatch in 6-7 days-> <------------Larvae swim up in 12-13 days------------> <----Larvae drift----> <-------------Larvae shift from endogenous to exogenous food source in 8-19 days-------------> <Larvae may starve without nursery habitat-> Figure 3-3. Chronology of egg incubation, swim-up phase, and shift to exogenous food sources for razorback sucker larvae. Larvae need a food source or may begin to starve at 8-19 days of age.